This invention relates to electronic devices, particularly but not exclusively with thin-film circuitry, comprising an array of device elements and having column multiplexer circuits. The device elements produce currents flowing in the columns of the arrays, and may be image sensor elements, for example photosensitive diodes, of a large area image sensor, or other types of device element, for example temperature sensing elements of a thermal imaging device.
Published European Patent Application EP-A-0 633 542 (Our reference PHB 33858) discloses such an electronic device comprising an array of device elements which are arranged in rows and columns and which are coupled to row and column conductors. The column conductors are arranged in groups, each group having a respective common terminal. A column multiplexer circuit couples the column conductors of a respective group to the respective common terminal. FIG. 3 of EP-A-0 633 542 illustrates a particularly advantageous form of such a device. The device is an image sensor array, the device elements of which comprise photosensitive diodes 8a and switching diodes 8b. The column conductors 10 transmit analogue signals from the photosensitive diodes 8a to a charge-sensitive amplifier 20 via the column multiplexer circuits. The column multiplexer circuits comprise photosensitive diodes 11b and 11c which are switched from a blocking state to a conductive state by illumination from a corresponding arrangement of light sources. Thus, the invention described and claimed in EP-A-0 633 542 permits the column multiplexer circuits to be fabricated with diodes and using the same technology type as the device elements of the array. FIG. 3 shows a group of three such column conductors 10 coupled by these column multiplexer circuits to one charge-sensitive amplifier 21. However, the column multiplexer circuits of EP-A-0 633 542 require alignment of the light sources with the photosensitive diodes of the multiplexer circuits. Depending on the type of light source used, the switching of the light source may also introduce an undesirable delay in the switching of the multiplexer circuits.
U.S. Pat. No. 4,518,921 discloses a sample and hold circuit comprising a diode bridge having first and second arms between first two current sources, each arm comprising a respective pair of diodes coupled together at a respective node of that arm and having the same polarity as each other between the current sources. The input of the sample and hold circuit is coupled to the node of the first arm, and the output is coupled to the node of the second arm. Control lines apply switching voltages for switching the two current sources on in a first state of the bridge and off in a second state of the bridge, a signal being sampled in the first state of the diode bridge. In the first, ON, state of the diode bridge, the current sources supply current through the diodes of the diode bridge, and the current sources are supplied with appropriate voltages to enable this.
The use of current sources to supply and drain the diode bridge of U.S. Pat. No. 4,518,921 enables the voltage levels of the diode bridge to float so that the output voltage may follow the input voltage. The sample and hold circuit further comprises second two current sources connected in parallel with the first two current sources, which provide a current through a third arm having a pair of diodes of opposite polarity to the diodes of the diode bridge. In this way, when the bridge is OFF, the second two current sources are turned on and a current flows through the third arm. The third arm serves to maintain the voltage across the bridge constant during the OFF interval, but without fixing the input voltage level. This is achieved by driving a constant current through fixed resistors. During the ON interval, the output voltage is free to follow the input voltage because the output is coupled to a capacitive load which charges or discharges as the output voltage follows the input voltage.
The invention seeks to provide a array of device elements having a column multiplexer circuit, the operation of the circuit using electrical switching rather than optical switching. The invention resides in the use of current-source fed diode bridge circuits as the switching elements of a multiplexing circuit for interfacing a number of column terminals of an array of electrical elements to a single terminal. This produces an electrical device according to which each of a number of column terminals has a sampling circuit, only one of which is turned on (in the first state of the bridge) so as to carry out a multiplexing operation.
A problem which arises in image sensors is interference between pixels in a column (vertical cross talk). This arises because currents are produced by illuminated pixels, and for a selected row these currents are free to flow into the associated column. When the associated column switch is turned off there is no path along which these currents may dissipate, and they accordingly create voltage biases on the pixels of other rows in the column.
Therefore, a more specific aim of the invention, in its first aspect, is to reduce vertical cross talk in such an array, by dissipating these currents. The sample and hold circuit of U.S. Pat. No. 4,518,921 could not achieve this, since in the OFF condition a fixed current is driven through the third arm, so no additional current may be accommodated. The diodes of the diode bridge are reverse biased and therefore also do not conduct any current.